The human skin is man's largest organ and performs a number of vital functions. Having an average area of about 2 m2 in adults, human skin plays a prominent role as a protective and sensory organ. The purpose of this organ is to transmit and avert mechanical, thermal, actinic, chemical and biological stimuli. In addition, human skin plays an important role as a regulatory and target organ in human metabolism.
The main objective of skin care in the cosmetics sense is to strengthen or rebuild the skin's natural function as a barrier against environmental influences (e.g. dirt, chemicals, microorganisms) and against the loss of endogenous substances (e.g. water, natural fats, electrolytes), and also to assist the horny layer of the skin in its natural regeneration ability where damage has occurred.
If the barrier properties of the skin are impaired, increased resorption of toxic or allergenic substances or infection by microorganisms may result, leading to toxic or allergic skin reactions.
Another aim of skin care is to compensate for the loss by the skin of sebum and water caused by daily washing. This is particularly important if the natural regeneration ability of the skin is inadequate. Furthermore, skin care products should protect the skin against environmental influences, in particular against the effects of sun and wind, and delay skin ageing.
Medicinal topical compositions usually comprise one or more medicaments in an effective concentration. For the sake of simplicity, in order to clearly distinguish between cosmetic and medicinal use and corresponding products, reference is made to the legal provisions in the Federal Republic of Germany (e.g. Cosmetics Directive, Foods and Drugs Act).
Emulsions are generally understood to be heterogeneous systems which consist of two liquids which are immiscible with one another or which are miscible with one another only to a limited extent, and which are usually referred to as phases. In an emulsion, one of the two liquids is dispersed in the form of very fine droplets in the other liquid.
If the two liquids are water and oil and oil droplets are very finely dispersed in water, this is an oil-in-water emulsion (O/W emulsion, e.g. milk). The basic character of an O/W emulsion is determined by the water. In the case of a water-in-oil emulsion (W/O emulsion, e.g. butter), the principle is reversed, the basic structure being determined here by the oil.
The person skilled in the art is of course aware of a large number of ways to formulate stable W/O preparations for cosmetic or dermatological use, for example in the form of creams and ointments which can be spread in the temperature range of from room temperature to skin temperature, or as lotions and milks, which are more likely flowable in this temperature range. However, the prior art recognizes only a few formulations which are of such low viscosity that they would, for example, be sprayable.
In addition, low-viscosity preparations of the prior art often have the disadvantage that they are unstable and are limited to a narrow range of application or to a restricted choice of starting materials. Low-viscosity products in which, for example, strong polar oils—such as the vegetable oils frequently used in commercially available products—are sufficiently stabilized cannot therefore be currently found on the market.
W/O emulsions with a high water content and a low viscosity, which moreover have a storage stability which is required for marketable products can only be formulated according to the prior art in a very complex manner. Accordingly, the supply of such formulations is extremely low. Nevertheless, such formulations could offer the consumer cosmetic effects which are hitherto unknown.
The object of the present invention was to provide water-in-oil emulsion based preparations which have very low viscosity and which do not have the disadvantages of the prior art.
A further object of the present invention was to provide preparations which can be loaded with a high content of water-soluble and/or water-miscible substances having cosmetic or dermatological effectiveness, without impairing the galenical quality or other properties of the preparations.
According to K. J. Lissant: The Geometry of High-Internal-Phase-Ratio Emulsions; Journal of Colloid and Interface Science 22, 462–468 (1966), “high internal phase emulsions” are defined as emulsions with an internal phase of >70%.
In particular, the preparation of stable, flowable water-in-oil emulsions having a water content of more than 70% has proven to be very difficult. In particular, “high internal phase” W/O emulsions with a very high water content of more than 85% (“very high internal phase” W/O emulsions) are not accessible.
The preparation of stable, solid to flowable “high internal phase water-in-oil emulsions”, in particular those with a water content higher than 80% by weight (“very high internal phase water-in-oil emulsions”) and nevertheless with very good sensory properties is an unsolved problem. As a result of the very high water content in the emulsions, the latter “crack” on the skin particularly rapidly (sensorially unpleasant) into their main constituents (hydrophilic and lipophilic components). Furthermore, the lipophilic components also separate into their individual constituents, meaning that the lipids “slide away” from one another on the skin (sensorially unpleasant).
the technique of varying the phase/volume ratio (i.e. of incorporating higher amounts of liquid lipids), which is usually used for water-in-oil emulsions, can, because of the low lipid content, be used only to a limited extent in the case of “high internal phase” W/O emulsions, and not at all in the case of “very high internal phase” W/O emulsions. It is therefore only possible to obtain water-in-oil emulsions with a solid to semi-solid consistency. Even the use of polar lipids, which are usually used to obtain water-in-oil emulsions of lower viscosity, does not lead to the desired success.